Support completion in enum constant argument list.

Change-Id: Id483d6ea3715e567f1d2cc9a2051f615b750a8a9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/232684
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analysis_server/test/services/completion/dart/arglist_contributor_test.dart

@@ -738,6 +738,16 @@ void f() {}
 ''');
     await computeAndCheck();
 
+    // Enum constant.
+    addTestSource2('''
+$languageVersionLine
+enum E {
+  v$arguments;
+  const E$parameters;
+}
+''');
+    await computeAndCheck();
+
     // Function expression invocation.
     addTestSource2('''
 $languageVersionLine

pkg/analyzer_plugin/lib/src/utilities/completion/completion_target.dart

@@ -291,6 +291,12 @@ class CompletionTarget {
       Element? executable;
       if (invocation is Annotation) {
         executable = invocation.element;
+      } else if (invocation is EnumConstantArguments) {
+        var enumConstant = invocation.parent;
+        if (enumConstant is! EnumConstantDeclaration) {
+          return null;
+        }
+        executable = enumConstant.constructorElement;
       } else if (invocation is InstanceCreationExpression) {
         executable = invocation.constructorName.staticElement;
       } else if (invocation is MethodInvocation) {

pkg/analyzer_plugin/lib/src/utilities/completion/optype.dart

@@ -1376,6 +1376,8 @@ class _OpTypeAstVisitor extends GeneralizingAstVisitor<void> {
       var parent = node.parent;
       if (parent is Annotation) {
         return 'annotation';
+      } else if (parent is EnumConstantArguments) {
+        return 'enumConstantArguments';
       } else if (parent is ExtensionOverride) {
         return 'extensionOverride';
       } else if (parent is FunctionExpressionInvocation) {